A powerful mudslide struck Gyirong County in southwest China's Xizang Autonomous Region on August 26 . The powerful geohazard also blocked the transboundary river channel, creating a dangerous barrier lake and adding severe secondary disaster risks to ongoing rescue operations.

What is a barrier lake?

A barrier lake is a typical secondary geohazard that forms when a river channel is blocked by loose rock, soil, ice or debris from a landslide, debris flow or ice-rock avalanche, creating a natural barrier dam. Water then accumulates behind the unstable barrier, forming a temporary lake.

Unlike man-made concrete dams, barrier dams are generally made up of loose, unconsolidated materials and can be vulnerable to overtopping or sudden breach, potentially triggering flooding downstream.

In the current case, the river channel was blocked by landslide debris, causing water to accumulate behind the natural barrier.

Preliminary measurements by the rescue team show that the barrier is about 150 meters long and 40 to 50 meters high, with the lake's storage capacity estimated at between 1.5 million and 2 million cubic meters.

According to China's Ministry of Water Resources, as of Thursday morning, the lake contained about 2 million cubic meters of water and had reached an overflowing state. Another 3 million cubic meters of water is expected to flow into the lake over the next three days.

Formation of the current Gyirong barrier lake

The disaster began with a high-altitude ice collapse in the upper reaches of Cuojian, a tributary of the Donglin Zangbo River in Nepal, according to China’s Ministry of Natural Resources. The falling ice scoured massive amounts of moraine deposits along the steep valley, forming a high-speed debris flow that rushed downstream and struck Gyirong Port in southwest China’s Xizang Autonomous Region.

Large volumes of ice-rock avalanche deposits and subsequent local landslides blocked the confluence of the Puripu Qiangzangbu River and the Cuojian River. Continuous upstream river water was trapped behind the natural dam, gradually building up water storage and forming the cross-border barrier lake.

By the morning of  August 27, the lake held an estimated 2 million cubic meters of water and had reached an overtopping state. Hydrological forecasts project an additional inflow of 3 million cubic meters over the following three days, pushing up breach-flood risks sharply. 

As of August 28, overtopping flow continues and rescue missions heading to the core disaster zone have been temporarily suspended for safety reasons.